1. Field of the Invention
The present invention relates to an image processing method for receiving a color image in the form of digital signals and effecting a pseudo-halftone processing to display said signals or convert said signals into recording signals.
2. Related Background Art
In a color image processing apparatus such as a digital color copying machine, a pseudo-halftone processing has been conducted on the image data, obtained by reading an original image, for image display or image recording. For such pseudo halftone processing there are already known, for example, the dither method and the error diffusion (ED) method.
For reproducing a color image with these methods, for example with a light emitting display such as a cathode ray tube, the input multi-value data of red, green and blue color components are subjected to pseudo-halftone processing independently in the color spaces of red, green and blue. Also, in the case of using a thermal recording apparatus such as a thermal transfer printer, an ink jet recording apparatus or an electrostatic recording apparatus, the input multi-value data are generally transformed from the R, G, B-color coordinate system to the C, M, Y (K)-color coordinate system and are subjected to independent pseudo halftone processing in respective color spaces as explained above.
However the pseudo halftone processing of density preserving type, such as the error diffusion method, is basically designed to conserve the density in a microspace in each color space, so that the color synthesized from the results of independent processings in three or four color spaces does not necessarily coincide with the color of the original image. Stated differenly, each point on the recording sheet does not necessarily have exactly the color of that point of the original input image. Particularly small black dots resulting from black color generation in human skin color areas give rise to an unnatural texture, thus resulting in the deterioration of the image quality.
Also, in the case of image production with an apparatus utilizing the RGB color space and that utilizing the CMYK color space, there is required a color coordinate converting circuit from the RGB color space to the CMYK color space, or vice versa.
Furthermore, in the case of variation in the recording characteristics of a binary recording apparatus, the correction of so-called masking or undercolor removal coefficients requires an enormous amount of data processing which cannot be achieved inexpensively in a practical apparatus. Also, the compression of redigitized data after pseudo-halftone processing can only be effected independently in each of four colors.